Carleton: What happened to the four-man rotation?

From Russell Carleton at Baseball Prospectus on December 16, 2013:

Lately, I’ve been wondering about the development of the modern pitching staff. I’ve looked at how we got to the point where no one completes a game anymore and why pitch counts have fallen over the years. Here’s another. What happened to the four-man starting rotation? It used to be that a team had four starters, each of whom pitched on three days’ rest…or so the story goes. There were always days off and travel days, and then there were doubleheaders, so there were swingmen who picked up the occasional start. While we can’t yet be sure what happened, we at least have an idea of when it happened. Here’s a chart showing the percentage of starts featuring a pitcher who was on three days of rest (or fewer) from 1950 to 2012.

<snip>

We see that in the 1950s and ’60s, pitchers were going on what is now considered “short” rest more than 40 percent of the time. In 1975, the rates began a rather quick decline and have reached the point where it’s a news story when a pitcher is going on three days’ rest. A five-man rotation (occasionally six) is now the norm.

Not only that, but using the pitch estimator that I constructed a couple of weeks ago, I looked at the number of (estimated) pitches that the pitcher threw in the start before this one, based on whether the start took place with four calendar days’ rest (or more) vs. three calendar days’ rest (or less). While in recent years, pitchers—on the rare occasions that they do pitch on three days’ rest—are more likely to do so only after a short outing, that wasn’t always the case. Before 1975, the lines are essentially running parallel. It used to be that pitching on three days’ rest was just something you did. Now, it’s something that you do only when necessary.